Copolymers of vinyl halides and vinyl esters such as vinyl chloride/vinyl acetate copolymers are well known in the art and have found wide acceptance in the marketplace.
The demand for specialty polyvinyl chloride resin for use in custom injection molding (CIM) applications initiated research for PVC resins having high melt flow and low molecular weight, without any corresponding adverse effects on heat stability, heat distortion and related properties. The use of low levels of vinyl acetate as a copolymer for PVC substantially improves the melt flow properties, but there is a further need to reduce the molecular weight of the polymer. The molecular weight reduction is accomplished through the practice of this invention by the use of a mercapto compound as a chain transfer agent.
One of the most efficient chain transfer agents in the family of mercaptans is 2-mercaptoethanol. It is more effective than other conventional chlorinated chain transfer agents, such as trichloroethylene. U.S. Pat. No. 4,189,552, which is herein incorporated by reference, discloses the use of 2-mercaptoethanol in the suspension copolymerization of vinyl chloride and vinyl acetate with 2-mercaptoethanol levels of 0.001 to 0.50 parts per 100 parts of monomer. The '552 patent teaches that the introduction of the mercapto compounds into the reaction medium at a time when the monomer conversion is lower than 1% produces adverse effects on particle characteristics. Because of the detrimental effects that mercapto compounds have on colloidal stability, the '552 patent instructs that the mercapto compounds should be introduced by divided addition during the polymerization. In essence, 2-mercaptoethanol at levels greater than 0.03part per 100 parts monomer, charged before the onset of the polymerization, is likely to cause an unacceptable coarse or solid charge.
It is a desirable goal to be able to use highly efficient mercaptan chain transfer agents, such as 2-mercaptoethanol, at high levels and without resorting to step wise addition while maintaining the colloidal stability of an aqueous polymerization reaction.